OP0595, a new diazabicyclooctane: mode of action as a serine β-lactamase inhibitor, antibiotic and β-lactam 'enhancer'.

Objectives: The production of a growing diversity of beta-lactamases by Gram-negative bacteria challenges antimicrobial chemotherapy. OP0595, discovered separately by each of Meiji Seika Pharma and Fedora Pharmaceuticals, is a new diazabicyclooctane serine beta-lactamase inhibitor that also acts as an antibiotic and as a beta-lactamase-independent beta-lactam 'enhancer'. Methods: Inhibitory activity against serine beta-lactamases and affinity for PBPs were determined using nitrocefin and Bocillin FL, respectively. MICs alone and in combination with beta-lactam agents were measured according to CLSI recommendations. Morphological changes in Escherichia coli were examined by phase-contrast microscopy. Results: IC(50)s of OP0595 for class A and C beta-lactamases were <1000 nM, with covalent binding demonstrated to the active-site serine of CTX-M-44 and AmpC enzymes. OP0595 also had direct antibiotic activity against many Enterobacteriaceae, associated with inhibition of PBP2 and conversion of the bacteria into spherical forms. Synergy between OP0595 and beta-lactam agents was seen against strains producing class A and C beta-lactamases vulnerable to inhibition. Lastly, OP0595 lowered the MICs of PBP3-targeted partner beta-lactam agents for a non-beta-lactamase-producing E. coli mutant that was resistant to OP0595 itself, indicating beta-lactamase-independent 'enhancer'-based synergy. Conclusions: OP0595 acts in three ways: (i) as an inhibitor of class A and C beta-lactamases, covalently binding at their active sites; (ii) as an antibacterial, by inhibiting PBP2 of several Enterobacteriaceae; and (iii) as an 'enhancer' of beta-lactam agents that bind to other PBPs besides PBP2 for several Enterobacteriaceae. OP0595 has considerable potential to overcome resistance when it is combined with various beta-lactam agents.